In general, a thermosensitive recording medium has a thermosensitive layer containing a color former such as leuco dye, a developer such as a phenol compound, and a binder resin such as polyvinyl alcohol on a substrate such as paper, a plastic film, etc. In the thermosensitive recording medium, the color former reacts with a heated and melted developer, and develops colors, thereby desired characters, images, etc. are formed.
The thermosensitive recording medium can be printed using a thermosensitive printer which is commonly compact and makes minimum noise during printing. Therefore, recently, the thermosensitive recording medium is used for very various applications such as facsimile paper, receipts, train tickets, price tags for groceries and prepared food, bills for gas or water, etc.
However, conventional thermosensitive recording media had significant problems in practical use in that characters, etc. printed on the thermosensitive recording medium are faded or disappear in time due to external factors. Therefore, environmental conditions when storing should be carefully noted.
Specifically, when developed parts of characters, etc. printed in the thermosensitive recording medium come in contact with plastic goods, a plasticizer, etc. contained in the plastic goods transfers into the thermosensitive layer constituting the thermosensitive recording medium, and causes color fading of the characters, etc. Such problems are also caused when a food label which is a thermosensitive recording medium, contacts with a common food wrapping film for a certain period of time.
Due to this, it is difficult to maintain clear characters, etc. for a sufficiently long period of time in practical use without color fading of characters, etc. printed thermally in conventional thermosensitive recording media.
Solutions for these problems have been examined. For example, a method in which a protective layer is formed on the thermosensitive layer in the thermosensitive recording medium is known. By using this method, transfer of plasticizers, etc. to the thermosensitive layer is prevented. As a result, it is possible to prevent color fading of characters, etc. thermally printed.
Therefore, among the thermosensitive recording media available commercially these days, most of the thermosensitive recording media used in applications which need to prevent color fading of characters, etc. have a protective layer containing polyvinyl alcohols, etc. as a main component.
Although it is expected that the applicable scope of the thermosensitive recording medium will become further expanded, development of a thermosensitive recording medium with high added value is also desired by the industrial arena.
For example, the thermosensitive recording medium used as an admission ticket of a concert is desired to have non-thermosensitive color developing parts, where clear hue pictures, patterns, etc. are printed, for the purpose of addition of high design and appeal of commercial goods to consumers. In general, most of the pictures, patterns, etc. are printed on the protective layer. An offset printing method, which has been known as a method by which pictures having high level sharpness can be printed, is desired to be used as the printing method.
Offset printing method is a method in which a hydrophobic ink is attached to a plate reflected with images to be printed, then the ink attached to the plate is transferred to a rubber roller generally called a “blanket”, and the ink attached to the blanket is adhered to paper to obtain printed material. The plate has printed areas which attach an ink and non-printed areas which do not attach an ink. The non-printed area is generally made wet with dampening water in order to prevent attachment of a hydrophobic ink.
However, most of the protective layer in the known thermosensitive recording media contains water-soluble substances such as polyvinyl alcohol as a main component. Therefore, the dampening water, which may be adhered to the protective layer while the protective layer is offset printed, sometimes causes various problems.
For example, there is a case in which when the dampening water contacts with the ink, the dampening water causes emulsion of the ink. Since the emulsified ink does not readily spread or attach to the protective layer in the thermosensitive recording medium, there is a case in which printing defects such as partial printing omissions (known generally as the “mizumake phenomena”) are caused.
In addition, when polychrome printed material is obtained by an offset printing method, each color is generally printed repeatedly on the protective layer by the processes.
However, the dampening water attached on the protective layer by printing a first color ink prevents a second or later color ink from spreading and attaching on the protective layer. As a result, printing defects are sometimes caused.
Furthermore, the dampening water attached on the protective layer sometimes also causes elution of components constituting the protective layer. When the eluted component is deposited on the blanket, this sometimes causes printing defects (this is generally called “piling”).
A coating composition, which can produce a protective layer which does not readily cause printing defects, in other words, a protective layer having excellent offset printing suitability and color fading resistance of characters or the like printed thermally, has been examined.
For example, it is known that a protective layer, which contains at least one compound selected from the group consisting of high fatty acid amides, substituted modified products thereof, and aromatic bisamides, and polyvinyl alcohol and have a specific bekk smoothness, has superior offset printing suitability (For example, Patent Document No. 1).
In addition, a method for obtaining excellent offset printing suitability by producing a protective layer having wet-ability to flaxseed oil which has comparable surface tension to that of printing inks has been known. As a resin composition capable to produce a protective layer, a resin composition containing a water-soluble polymer such as polyvinyl alcohol, a cross-linking agent such as dimethyolurea, etc. is exemplified (For example, Patent Document No. 2).
Furthermore, as a thermosensitive recording medium having an overcoat layer (protective layer) which has excellent water resistance and stampability to oil based inks, a thermosensitive recording medium having an overcoat layer containing a pigment, amorphous silica, a cross-linking agent, and 0.5 to 2.0 parts by weight of carboxy-modified polyvinyl alcohol with respect to 1 part by weight of the pigment has been known (For example, Patent Document No. 3).
Although the protective layer produced using the resin composition disclosed in Patent Documents Nos. 1 and 2 has excellent offset printing suitability on some level, the suitability is not still satisfactory. In addition, since it is difficult to prevent sufficiently the transfer of the plasticizer, etc., it is also difficult to decrease color fading or disappearing of characters, etc. printed on the thermosensitive recording medium to a sufficient level in practical use.
In addition, the overcoat layer produced using the composition disclosed in Patent Document No. 3 has superior stampability oil based inks, but this does not still have excellent offset printing suitability.
As explained above, in order to improve properties of the protective layer containing a water-soluble resin such as polyvinyl alcohol, several methods of containing both a cross-linking agent, a pigment, etc. have been studied. However, conventional techniques could not produce a protective layer in a thermosensitive recording medium, which has both excellent offset printing suitability and color fading resistance of characters, etc. Development of a modifying agent which can provide these excellent properties to the protective layer is still desired by the industrial arena.    Patent Document No. 1: Japanese Unexamined Patent Application, First Publication No. H04-074686    Patent Document No. 2: Japanese Unexamined Patent Application, First Publication No. H04-082777    Patent Document No. 3: Japanese Unexamined Patent Application, First Publication No. 2000-289333